10 Steel Structure Welding Tips You Should Know

1. Advantages and disadvantages of welding connection?

Advantages of welding connection: 

Simple structure, no weakening of component section, simple processing, various welding methods, automatic operation, saving steel, high efficiency, large stiffness, good integrity, and good sealing performance.

Disadvantages of welding connection: 

The metallographic structure of steel in the heat-affected area changes and local materials become brittle;

After welding, there are welding residual stress and residual deformation, which reduce the bearing capacity of compression members;

The welded structure is very sensitive to cracks.

Once local cracks occur, they are very easy to extend to the whole, and low-temperature cold brittleness is more prominent.

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2. Definition of steel weldability and influencing factors?

The weldability of steel refers to the degree to which materials are easy to weld and meet the structural performance under appropriate design and working conditions.

Weldability is often affected by chemical composition of steel, rolling method and plate thickness.

In order to evaluate the influence of chemical composition on weldability, it is generally expressed by carbon equivalent (Ceq).

The smaller the Ceq is, the smaller the hardening tendency of steel is, and the better the weldability is;

On the contrary, the larger the Ceq, the greater the hardening tendency of the steel, and the worse the weldability.

The carbon equivalent Ceq (percentage) value can be calculated according to the following formula:

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3. What are the causes of welding stress and welding deformation and how to reduce them?

The welding process of steel structure is a process of uneven heating and cooling.

The temperature of the weld and its vicinity is very high during welding, while most of the metal in the distance is not heated, and the expansion and contraction of the main metal are uneven.

After cooling, the weld seam will produce different degrees of shrinkage and internal stress (longitudinal and transverse), resulting in various deformations of the welding structure.

Generally speaking, welding stress and welding deformation can be reduced from two aspects of design and processing technology:

Design measures:

Reasonably arrange the weld position;

Reasonable selection of weld size;

The number of welds should be small and should not be too concentrated, and the three-dimensional crisscross of welds should be avoided;

Shrinkage stress in the thickness direction of base metal shall be avoided as far as possible.

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Process measures:

Arrange welding sequence reasonably;

Adopt reverse deformation;

Preheating before welding and tempering after welding.

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4. Common welding methods for steel structures?

The commonly used welding methods for steel structures include manual arc welding, automatic (or semi-automatic) submerged arc welding and gas shielded welding.

Related reading: Manual Arc Welding vs CO2 Gas Shielded Welding

Manual arc welding:

After being electrified, an electric arc is generated to melt the welding wire in the electrode and drop it into the small groove molten pool blown by the electric arc on the weldment.

The slag and gas formed by the electrode coating cover the molten pool to prevent the air from contacting the molten liquid metal and the formation of brittle and crack prone compounds.

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  • a) Circuit:
  • b) Welding process;
  • 1 – Electric welding machine:
  • 2 – Conductor;
  • 3 – Weldment;
  • 4 – Arc:
  • 5 – Coated skin;
  • 6 – Gas for protection;
  • 7 – Slag:
  • 8 – Weld metal;
  • 9 – Base metal;
  • 10 – Welding wire;
  • 11 – Molten pool

Submerged arc welding:

An arc welding method in which the arc burns under the flux layer.

The welding wire is not coated with coating, but the welding end is covered by the granular flux flowing automatically from the flux leakage head.

The arc is completely buried in the flux. The heat of the arc is concentrated and the penetration is large. It is suitable for welding thick plates with high productivity.

At the same time, the welding quality is good and the deformation of the weldment is small.

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  1. Welding wire turntable
  2. Wire feeding motor
  3. Flux funnel
  4. Power supply
  5. Melted flux
  6. Weld metal
  7. Weldment
  8. Flux
  9. Moving direction

Gas shielded welding:

An arc fusion welding method using carbon dioxide or other inert gases as the protective medium.

The local protective layer is formed around the arc by the shielding gas to prevent the invasion of harmful gas and ensure the stability of the welding process.

The weld strength is higher than that of manual arc welding, with good plasticity and corrosion resistance.

It is suitable for all position welding, including forward method and backward method.

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5. Common welding codes?

Common welding positions, joint forms, groove forms, weld types and pipe structure node form codes are shown as follows:

CodeWelding position
Fdownward welding
HHorizontal position welding
VVertical welding
OOverhead position welding
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Related reading: What Does 1G, 2G, 3G, 4G, 5G and 6G Mean in Welding?

Joint type and groove shape code

Type of joint

 

Groove shape

 

Code

Name

Code

Name

I

I-groove

Plate joint

B

Butt joint

V

V-groove

T

T-joint

X

X-groove

X

pipe cross

L

Single side V-groove

C

fillet joint

K

K-groove

F

Lap joint

U1

U-groove

Pipe joint

 

 

T

T-joint

J1

Single side U-groove

K

K-joint

Note:
1. When the thickness of steel plate is ≥ 50mm, U-shaped or J-shaped groove can be used

Y

Y-joint

6. Common welding defects, causes and treatment methods?

The weld defects are divided into six categories: cracks, cavities, solid inclusions, incomplete fusion, incomplete penetration, and shape defects.

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Crackle:

There are usually hot cracks and cold cracks.

The main causes of hot cracks are poor crack resistance of base metal, poor quality of welding materials, improper selection of welding process parameters, excessive welding internal stress, etc;

The main causes of cold cracks are unreasonable design of welding structure, improper arrangement of welding seams, and unreasonable welding process measures, such as no preheating before welding, fast cooling after welding, etc.

The treatment method is to drill crack stop holes at both ends of the crack or remove the weld metal at the crack for repair welding.

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Cavity:

It is generally divided into two types: air hole and crater shrinkage.

The main reasons for air holes are that the coating of the electrode is seriously damaged, the electrode and flux are not baked, the base metal has oil stain or rust and oxide, the welding current is too small, the arc length is too long, the welding speed is too fast, etc.

The treatment method is to shovel the weld metal at the air hole, and then repair welding.

The main reasons for crater shrinkage are that the welding current is too high, the welding speed is too fast, the arc quenching is too fast, and the filler metal is not supplemented to the arc quenching repeatedly.

The treatment method is to repair welding at the crater.

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Solid inclusion:

There are two defects: slag inclusion and tungsten inclusion.

The main reasons for slag inclusion are poor quality of welding materials, too low welding current, too fast welding speed, too high slag density, blocking the floating of slag, and the slag is not cleaned up during multi-layer welding.

The treatment method is to remove the weld metal at the slag inclusion and then weld.

The main reason for tungsten inclusion is that the tungsten electrode contacts the molten pool metal during argon arc welding.

The treatment method is to dig out the defective metal at the tungsten inclusion and weld again.

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Incomplete fusion and penetration:

The main reasons are that the welding current is too small, the welding speed is too fast, the groove angle gap is too small, and the operation technology is poor.

The treatment method for non fusion is to remove the weld metal at the non fusion position and then weld repair.

The treatment method for incomplete penetration is to repair the incomplete penetration on one side of the structure with good openness directly at the back of the weld.

For important weldments that cannot be directly repaired by welding, the incomplete weld metal shall be removed and re welded.

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Shape defect:

Including undercut, overlap, sagging, root shrinkage, misalignment, angle deviation, weld superelevation, surface irregularity, etc.

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7. Common measures to prevent lamellar tearing of plates?

In T-shaped, cross shaped and corner joints, when the thickness of flange plate is not less than 20mm, in order to avoid or reduce large welding shrinkage stress in the direction of base metal plate thickness, the following joint structure design should be adopted:

Under the condition of meeting the requirements of penetration depth and weld tightness, smaller welding groove angle and gap (a) shall be adopted;

In the corner joint, symmetrical groove or groove inclined to the side plate (b) is adopted;

Symmetrical welding of double-sided groove is adopted instead of asymmetric welding of single-sided groove (c);

In a T-shaped or corner joint, the end of the plate bearing welding tensile stress in the direction of plate thickness extends out of the weld zone of the joint (d);

In T-shaped and cross shaped joints, cast steel or forged steel transition section is used, and butt joint is used to replace T-shaped and cross shaped joints (e, f);

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Change the stress direction of the thick plate joint to reduce the stress in the thickness direction;

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For nodes subject to static load, under the condition of meeting the requirements of joint strength calculation, the fully penetrated groove weld shall be replaced by the partially penetrated butt and fillet weld.

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8. Weld quality inspection method?

When the welding is completed and the weld inspection is carried out, the appearance inspection shall be carried out first, and the visual inspection or magnifying glass shall be used to observe whether there are defects, such as undercut, burn through, incomplete penetration, cracks, staggered edges, staying, etc., and whether the overall dimensions of the weld meet the requirements.

The flaw inside the weld is usually detected by ultrasonic wave, which is based on the principle that ultrasonic wave can spread inside the metal and reflect and refract when encountering the interface of two mediums to inspect the flaw inside the weld.

The existence and location of the flaw can be judged according to the waveform.

Since there is a reflective surface between the probe and the test piece, the coupling agent should be coated on the surface of the weldment during ultrasonic inspection, and the ultrasonic cannot judge the type and size of defects.

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Radiographic inspection is also sometimes used in nondestructive testing.

There are two types of radiographic inspection: X-ray inspection and γ-ray inspection.

The principle is that when the ray passes through the inspected weld, if there is a defect, the ray passing through the defect will have less attenuation.

Therefore, the negative film on the back of the weld is highly sensitive to light.

After the negative film is washed, black spots or stripes will appear at the defect.

The X-ray exposure time is short, the speed is fast, the equipment is complex, the cost is large, the penetration capacity is small, and the thickness of the tested weldment is less than 30mm.

γ radiographic inspection equipment is portable, easy to operate, and has strong penetration capacity.

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9. What is the basis for judging the results of sampling inspection?

When the unqualified rate of the number of welds in the sampling inspection is less than 2%, the batch is accepted;

When the unqualified rate of the number of welds in the sampling inspection is more than 5%, the batch is unacceptable;

Except for the situation in the fifth paragraph of this article, when the unqualified rate of the number of welds in the sampling inspection is 2%~5%, the sampling inspection shall be doubled, and one extension line of the weld on both sides of the original unqualified part must be added.

When the unqualified rate of all the welds in the sampling inspection is not more than 3%, the batch is qualified;

When the unqualified rate is more than 3%, the batch is unqualified;

If the batch acceptance is unqualified, all the remaining welds of the batch shall be inspected;

If one crack defect is found in the inspection, double the spot check.

If no crack defect is found in the double spot check weld, the batch is accepted;

If multiple cracks are found during inspection or cracks are found after double spot check, the batch is unacceptable, and all the remaining welds in the batch shall be inspected.

10. What conditions require welding procedure qualification?

In addition to the conditions exempted from assessment in the national steel structure welding code, the combination conditions of steel, welding materials, welding methods, joint forms, welding positions, post welding heat treatment system, welding process parameters, preheating and post heating measures and other parameters adopted by the construction unit for the first time shall be subject to welding process assessment before the fabrication and installation of steel structure members.

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